Faculty: Bruce
Wooley
Student: Ali Tabatabaei
There is broad interest in A/D conversion at the intermediate frequency (IF) stage of a radio because the early front-end conversion to digital results in a more designable and testable system. Traditional analog signal processing such as I and Q separation and channel-select filtering can be moved into the digital domain, thereby avoiding the sensitivity of this process to various analogcircuit impairments. Furthermore, a digital demodulation of the signal allows the receiver to easily adapt to the multiplicity of standards that proliferate in the emerging wireless market. Unfortunately, as analog processing is eliminated, the signal that must be digitized prior to digital processing has larger dynamic range, and the converter must operate at a higher sampling speed. Also, in order to move the channel-select filter to the digital domain, a conversion bandwidth many times larger than a single channel bandwidth is required. CDMA is another application that requires large bandwidth. Analog to digital converters presently have insufficient dynamic range or conversion bandwidth, or consume too much power for these applications.
The objective of this research program is to demonstrate an analog to digital converter with sufficient dynamic range (>70 dB) and low enough power consumption (<100 mW) with a large signal bandwidth (>1.25MHz) for use in portable radios. The research is focused on an oversampled bandpass delta-sigma modulator based converter.
An oversampled converter is the clear choice, as these are the only type of converters that can obtain the large dynamic range necessary for wireless applications without requiring trimming or other steps incompatible with a low-cost CMOS process. Bandpass converters have several advantages. In this case, the converter digitizes a 1.25 MHz bandwidth signal. Compared to digitizing a baseband signal, this eliminates several analog signal processing steps, saving circuitry and power. If the signal had been digitized at baseband, two converters, one for the I channel and one for the Q channel, would have been required. Digitizing at an IF frequency also avoids problems with dc offsets and low frequency noise (1/f noise).
Since the maximum sampling frequency is limited by the power and precision of sampling circuits, large bandwidth requirement in this case demands for a relatively low oversampling ratio. To achieve a low oversampling ratio a higher order loop with multi-bit feedback is the most suitable choice. Currently, I am working on the architectural design.
A. Tabatabaei, K. Kaviani, B.A. Wooley, " A Two-Path Bandpass Sigma-Delta Modulator with Extended noise shaping ,"
ISSCC Digest of Technical Papers, pp. 342-343, Feb. 2000. Slides
A. Tabatabaei, B.A. Wooley, " A Wideband Bandpass Sigma-Delta Modulator for Wireless Applications ,"
IEEE Symp. on VLSI Circuits, pp. 91-92, June 1999.
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